Bentzen, Jens Dall

Brandt, Peder

Henriksen, Ulrik Birk

Hindsgaul, Claus

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Downloaded from orbit.dtu.dk on: Dec 18, 2017Optimized two-stage gascifierBentzen, Jens Dall; Henriksen, Ulrik Birk; Hindsgaul, Claus; Brandt, PederPublished in:Proceedings of 1st World Conference and Exhibition on Biomass for Energy and IndustryPublication date:2000Document VersionEarly version, also known as pre-printLink back to DTU OrbitCitation (APA):Bentzen, J. D., Henriksen, U. B., Hindsgaul, C., & Brandt, P. (2000). Optimized two-stage gascifier. InProceedings of 1st World Conference and Exhibition on Biomass for Energy and Industry Sevilla, Spain.Published by Kyritsis, S., Beenackers, A.A.C.M., Helm, P., Grassi, A., Chiaramonti, D. (eds) 2001,1st World Conference on Biomass for Energy and Industry. Proceedings of the Conferenceheld in Sevilla, Spain, 5-9 June 2000, London, James & James (Science Publishers) Ltd.1OPTIMISED TWO-STAGE GASIFIERJens Dall Bentzen*, Ulrik Henriksen, Claus Hindsgaul and Peder Brandt**Technical University of Denmark, Department of Energy Engineering, 2800 Lyngby, Denmark, uh@et.dtu.dk*COWI - Consulting Engineers and Planners AS, Parallelvej 15, 2800 Lyngby, Denmark, E-mail: jdb@cowi.dk**  Brandt-Product&Development, Kyringevej 47, DK-4100 Ringsted, Denmark, E-mail: bpu@post.tele.dkABSTRACT: The 100 kW two-stage gasifier at The Department of Energy Engineering of the Technical University ofDenmark (DTU) was reconstructed and hereby optimised in order to reduce the tar content in the produced gas and to reduceheat losses.Comparison of the tar level from the 100 two-stage gasifier before and after the reconstruction show that if very low tar contentin the produced gas are to be obtained, absolutely no gas shall be allowed to bypass the char bed.The tar content in the raw product gas was reduced to below 25 mg/Nm3, and heat losses were reduced about 60% due to thereconstruction.1. BACKGROUNDThe Department of Energy Engineering of the TechnicalUniversity of Denmark (DTU) has previously described thetwo-stage downdraft gasifier and reported its mainadvantages, namely low tar content and high energyefficiency [1].A 100 kW (thermal input) two-stage gasifier was built atthe Technical University of Denmark for furtherinvestigations. The gasifier was built in 1995 and wasequipped with gas cleaning system, automatic control anddatalogging in 1996.The 100 kW two-stage gasifier consists of a feedingsystem, an externally heated pyrolysis unit and a fixed bedgasifier (Figure 1). Two kinds of feeding systems wereconstructed: A screw feeder for wood chips and a lock-hopper for briquettes.Figure 1. The 100 kW two-stage gasifier at DTUOriginally, the gas cleaning system consisted of a hotcyclone, a gas cooler, a venturi scrubber and demistor, anda simple gas filter, but after an investigation of the particlesand after an optimisation of the tar reduction, the gascleaning system was reconstructed.2. PURPOSEBiomass gasifiers must have a high energy efficiency, havestable operation over long periods of time and must notpollute the environment in order to meet the requirementsof the market. It is, therefore, necessary to optimise theenergy efficiency and to reduce the tar content in theproduced gas as much as possible in order that gasificationcan be technically and economically viable.The purpose of this work was mainly to reduce the tarcontent in the gas from the 100 kW two-stage gasifier atDTU.3. OPTIMISATION OF THE 100 kW TWO-STAGE GASIFIERAfter 11 tests and investigations, the 100 kW gasifier wasinspected. The inner ceramic sealing was cracked in severalplaces so that some of the pyrolysis gasses, containing tars,could bypass the char bed and flow through the insulation(see Figure 2). The bypass resulted in high tar levels in theproduced gas.Gas cleaning systemSee Figure 62CHARBEDAirPartialoxidationProduct gasFigure 2. The 100 kW two-stage gasifier before theoptimisation. The bypass of the partial oxidation zone andof the char bed is illustratedIn Figure 2 the bypass of the partial oxidation zone and thebypass of the char bed are illustrated. Calculations showthat if 50 mg of tar/Nm3 is the highest acceptable level, andif the gas that flows though the char bed is tar free, lessthan 0,01% of the product gas can be allowed to bypass thechar bed.It was decided to put a high temperature metal sealing intothe reactor to ensure that all the gasses would flow throughthe char bed. Thus, the tar reduction should be higher, andthe convective heat losses should be reduced.Further, it was decided to reconstruct the top of the gasifierat the air inlet. New air nozzles were designed for bettermixing of pyrolysis gas and air, and better insulation of thehottest area of the gasifier was made to reduce heat losses.Figure 3. The optimised 100 kW two-stage gasifier atDTU, with high temperature metal sealing to ensure thatall the gasses would flow through the char bed; new airnozzles for better mixing of pyrolysis gas and air andbetter insulation of the hottest area.2 tests of the 100 kW two-stage gasifier after thereconstruction and several investigations have been made.The most important results of the investigations are brieflypresented in this paper. More detailed information on someof the investigations can be found in [2], [3], and [4].4. TAR IN PRODUCT GASThe tar content in the raw producer gas after the two-stagegasifier has been measured by several institutions after thereconstruction. The tar content is now extremely low, seeTable 1.Table. 1. The tar content in the produced  gas after thegasifier before filtering.Institution Method Date Tarcontentmg/Nm3Danish Tech-nologicalInstitute,DenmarkGravi-metricGCMS23 nov 199924 nov 199923 nov 199924 nov 19992525525TheTechnicalUniversity ofDenmark(DTU)GCMSGCMS9 sep 199810 sep 199823 nov 199924 nov 199916336RoyalInstitute ofTechnology,Sweden/DTUSolid phaseadsorption(SPA)8 measurements23-24 nov. 19995-18Average10Risø NationalLaboratory,DenmarkGCMS 9 sep 199810 sep 1998162,5It is seen in Table 1 that the tar content in the product gasnow is below 25 mg tar/Nm3.The tar content in the gas after the bag house filter wasmeasured two times by the Danish Technological Institutewith the results: 4 and 6 mg tar/Nm3.5. ENERGY EFFICIENCYCalculations of the heat loss (based on energy balance)before and after the optimisation show that the total heatlosses from the gasifier were reduced by about 60%, fromaround 11,0 kW to 4,3 kW.Based on the measured gas flow and heating value of thegas and on the fuel input, the cold gas efficiency (energy incold gas/energy in fuel) is calculated to be 92-97%. This is3an increase from  86-90% before the optimisation.Normally, low-tar gasification processes have a cold gasefficiency of about 80%.6. GAS CLEANINGTarget research has greatly improved the knowledge aboutthe origin and the physical and chemical properties of theparticles in the produced gas of the 100 kW two stagegasifier [3], [5]. This knowledge has resulted in theintroduction of a new strategy for the gas cleaning system.The physical properties of the particles were essentiallyunchanged by the reconstruction of the gasifyer. Only theparticle mass has increased from around 200-300 mg/Nm3to 500-800 mg/Nm3.Studies of the particles in the product gas have documentedthat the major part of the particles is soot.Soot is formed from the volatiles from the pyrolysisprocess in reducing atmosphere at high temperatures.The dominant particle size is between 0.2 and 0.5 µm, butsizes between 0.1–2 µm are observed. When the amount ofsteam is decreased, the amount of particles is increased.Typical particle size distributions measured by the gasifieroutlet are shown in Figure 4, and a SEM picture ofcollected particles is shown in Figure 5.Figure 4. Size distribution of the particles at differentbiomass steam ratios.Figure 5. SEM (Scanning Electron Microscope) picture ofparticles collected in the raw gas.The gas cleaning system was originally based on a venturiscrubber. Since better technologies exist for removingsubmicron particles, the gas cleaning system was rebuiltand is now based on a fabric bag filter (see Figure 6).GasifierCycloneHot cooler90ºC400ºC2nd coolerClear condensate45ºCFigure 6. The new gas cleaning system with a bagfilter that operates above the dew point.The results with the bag house filter were very good. Thefilter operates with a gas temperature of about 90°C - wellabove the dew point of the gas. Two different types of filtermaterials were tested with good results. The collectionefficiencies were above 95%. The regeneration of the filterswas very effective throughout the tests.Pressuredrop over bag house filter010020030012 15 18 21 00 03 06 09 12Pressuredrop (mmH2O)                      23/11 1999          Date and hours                         24/11 1999Figure 7 Pressure drop over bag house filter during one ofthe tests.In Figure 7, the pressure drop over the house filter during atest is shown. It is seen that the bags are easily regenerated.The acetone solubles ("tar") content in the particles was aslow as 3-5% by mass and did not seem to cause problemsin the filter bags. [3]. The particles were easy to removefrom the filter and had no tendencies to agglomerate orstick to surfaces.Since the particles are removed above the dew point, allcondensate in the following pipes has very low particlecontents. Compared to the scrubber system, the bag filterhas lower energy consumption, since its pressure drop ismuch lower. In addition, no water has to be added to thiscleaning system.7. CONDENSATEInvestigations of the condensate from the gasifiers showthat the condensate is so clean that it can be led to thepublic sewer without further treatment.4As regards inhibition of nitrifiers, the condensate of thetwo-stage gasifier can be led to the public sewer. Almostall the inhibition in the condensate of the two-stage gasifiercan be related to the ammonia content [6].The content of PAH and other organic compounds is verylow in the condensate of the two-stage gasifier after theoptimisation of the gasifier [2], [6].8. ACTIVATED CARBONSamples from different layers in the char bed bed havebeen analysed for the BET surface area. More detailedanalyses of the charcoal were made, and SEM (ScanningElectron Microscope) pictures of samples of the partialgasified char were taken, see Figure 9.The surface area was measured on samples from the sevenlayers from the char bed after a test in January 1998. TheBET surface area of the samples was between 800-1200m²/g. The molasses numbers (Europe) were 150-185, andthe Iodine numbers were 790-865. The conclusion of thetest from the laboratory was: “…It is obvious that theresults of the adsorption tests (molasses number etc.) showthat these samples have absorptive properties whichcertainly are comparable with a lot of activated carbons inthe market (powdered activated carbon)…..”Figure 9 Charcoal of the char bed from a test inJanuary 1998. It is seen how the pores of the charopen evenly while the main structure is maintained.9. CONCLUSIONSTar in product gas: The tar content in the product gas wasdramatically reduced due to the reconstruction. From tarmeasurements made by DTU, TI (Danish TechnologicalInstitute) and RISØ National Laboratory and KTH inStockholm, it can be concluded that the tar level in the rawgas is between 5-25 mg/Nm3 after the reconstruction foroptimisation.The tar content in the gas after particle filtration in a baghouse filter is about 5 mg tar/Nm3.Energy efficiency: Calculations of the heat loss (based onenergy balance) before and after the reconstruction showthat the total heat losses from the gasifier were reduced byabout 60%, from around 11,0 kW to 4,3 kW.The coldgas efficiency (energy in cold producer gas/energyin fuel) increased because of the lower heat losses frombetween 86-90% before the reconstruction to between 92-97% after.Gas cleaning: The particle content in the raw gas increaseddue to the reconstruction from around  200-300 mg/Nm3 toaround 500-800 mg/Nm3. The size distribution wasunchanged: Particles consist of soot agglomerates of sizesbetween 0,1-2 µm.The higher load of particles causes no problems for the gascleaning system.  Due to the very low tar content and thevery small particles [3], [5] it was decided to install a baghouse filter for particle filtration. The bag house filterfunctioned well with no increase in pulse frequency.Condensate: Investigations of the condensate from thegasifiers show that the condensate is so clean that it can beled to the public sewer without further treatment [2], [6].Activated carbon: Analyses and tests of charcoal from thetwo stage gasifier show that the charcoal has goodabsorptive properties.ACKNOWLEDGEMENTSThis work was financed by the Danish Energy Agency.REFERENCES [1]  U. Henriksen, O. Christensen. Gasification ofStraw in a Two-stage 50 kW Gasifier  October1994. Proceedings from "8th European Conferenceon Biomass for Energy, Environment, Agricultureand Industry". Vol. 2. pp. 1568-1578. Vienna.Austria. October 1994.[2] J. D. Bentzen and U. Henriksen. Condensate from atwo-stage gasifier.  1st World Biomass Conferenceand Technology Exhibition. 5-9 June, Sevilla,Spain.[3] C. Hindsgaul et. al. Chemical and physicalcharacterization of particles in producer gas fromwood chips. Bioresource Technology 73 (2000)p.147-155.[4] P. Brandt, E. Larsen, and U. Henriksen. High TarReduction in a Two-stage Gasifier. To be publishedin Energy&Fuels[5] C. Ravn, M. Rousing, 1998. “Dannelsen af Sod iForgasningsprocesser” Master thesis, Institut forKemiteknik. DTU, Denmark[6] J. la C. Jansen and K. Jönsson; Nitrificationinhibition of tar-water from wood chips gasifiers;1.st World Biomass Conference and TechnologyExhibition. 5-9 June, Sevilla, Spain.

Optimized two-stage gascifier

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Optimized two-stage gascifier

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